Responses to pure tone stimuli and model songs of ventral cord neurones of Corixa punctata Ill. (Heteroptera: Corixidae)

Abstract

The hearing organs of Corixa punctata males were stimulated with pure tones and model songs. Carrier frequencies of the stimuli ranged from 1 to 3 kHz, thus covering the peak frequencies of the conspecific song spectra. The spike responses of ventral cord neurones were recorded extracellularly. Four different neurone types were found on the right (R) side of the prothoracic ganglion and were characterized as follows: (1) Threshold curves of the 1.2−2.4 1 R neurones showed a double peak of best frequency sensitivity (Fig. 1). According to stimulus carrier frequency their response saturated or reached a maximum between 10 and 25 dB above threshold (Fig. 2A). Neurones copied different stimulus durations in a phasic-tonic way (Fig. 3A). Time structure of a model song, simulating the conspecific sound emission, was faithfully copied in the frequency range between 1.6 and 2.8 kHz (Fig. 3E). Neurone's responses habituated at the repetition rate of pulse trains typical for the conspecific song; the degree of habituation was determined by the carrier frequency and sound intensity of the pulses (Fig. 6). (2) Threshold curves of the 1.6−2.4 2 R neurones showed two peaks of best sensitivity but were 7–14 dB less sensitive than the 1.6−2.4 1 R neurones (Fig. 1). They responded only with an “on”-burst of spikes irrespective of stimulus carrier frequency and stimulus duration; the response magnitude was mainly determined by stimulus intensity (Figs 2B, 3B, 4A and B). Because of fast habituation they responded only to the first pulse train in a sequence with natural repetition rate (Fig. 3F). (3) Neurones of the 2.4 1 R type responded best at frequencies between 2.0 and 2.8 kHz (Fig. 1). Their intensity/response curves were bell shaped (Fig. 2C). Prolonged responses were typical for stimuli shorter than 20 ms (Fig. 3C). Pure tone stimuli of longer duration or high pulse repetition rates, as they occur in the natural songs, inhibited responses in these neurones (Figs 3C, 4C and D, 5). A pulse locked response pattern was well expressed only to the first two pulses in a pulse train (Fig. 3G). Carrier frequency and sound intensity determined the degree of response habituation to the natural repetition rate of pulse trains (Fig. 6). (4) Neurones of the 1.0 1 R type did not exhibit special tuning to the song spectra of Corixa. They responded best to acoustic stimuli with carrier frequencies below 1.6 kHz (Fig. 1). At higher frequencies they were about 20 dB less sensitive than the three other neurones. Pure tone stimuli were copied in a phasic-tonic way (Fig. 3D).